Electronic ballasts generally include an inverter that provides high frequency current for efficiently powering gas discharge lamps. Inverters are commonly classified according to switching topology (e.g., half-bridge or push-pull) and the method used to control commutation of the inverter switches (e.g., driven or self-oscillating). In many types of electronic ballasts, the inverter provides an output voltage that is processed by a resonant output circuit to provide a high voltage for igniting the lamps and a magnitude-limited current for powering the lamps.
Ballasts for gas discharge lamps provide high ignition voltages for starting the lamps. The ignition voltages supplied by preheat type ballasts are typically on the order of several hundred volts (e.g., 500 volts peak), while those provided by instant-start type ballasts may approach 800 volts peak. As a consequence of these high ignition voltages, ballasts are subject to a special type of lamp fault condition that is commonly referred to as output arcing.
Output arcing may occur in any of a number of different ways. For example, in fluorescent lighting installations, it is a common practice to replace failed lamps while AC power is applied to the ballast. This practice is referred to as “live” relamping. During live relamping, as a lamp is being removed or inserted, a momentary arc may form between the fixture socket contacts and a pin of the lamp. As another example, a sustained arc (as opposed to a momentary arc) may occur due to poor or faulty connections in the output wiring or the lamp sockets, or if a lamp is improperly installed in such a way that a small gap exists between the lamp pins and the contacts within the fixture sockets. If a connection to a lamp is compromised due to a defective lamp socket or defective wiring, a high intensity, high temperature arc may be produced across the air gap caused by those faulty connections.
Arcing is generally acknowledged to cause degradation of the contacts in the fixture sockets and undue stress on components within the ballast. Sustained arcing is especially undesirable because of its tendency to produce potentially destructive heating. In order to minimize any ill effects due to arcing, it is important that the arc be promptly extinguished. This requires a ballast that is capable of quickly and reliably detecting an arc and, subsequently, taking appropriate action to promptly extinguish the arc.
The prior art includes a number of circuits for detecting and/or protecting against output arcing, such as those which are disclosed in U.S. Pat. No. 6,720,739 B2 (Konopka) and U.S. Pat. No. 7,042,161 B1 (Konopka). The circuitry disclosed in both of those patents appears to represent a considerable advance over the prior art.
Many existing ballasts with arc protection circuits respond to an output arcing condition by shutting down the inverter and then keeping the inverter off for as long as power continues to be applied to the ballast. With such ballasts, following elimination of an output arcing condition, it is required that power to the ballast be turned off and then on again (i.e., “cycled”) in order to effect ignition and powering of the lamps in the fixture. This requirement poses a considerable inconvenience in many applications, such as in large office areas or factories, in which a large number of ballasts are often connected in the same branch circuit. In such environments, with many existing ballasts, it is necessary to momentarily interrupt the lighting in a large area in order to restore desired operation to even a single lighting fixture after one or more of its lamps are replaced. It is thus desirable to have a ballast that accommodates relamping without requiring that the power to the ballast be removed and reapplied.
It is also important that arc detection be inhibited during certain operating periods, such as inverter startup and lamp ignition. For instance, the normal starting process of the inverter and lamps is generally accompanied by the same types of electrical disturbances that occur during an arc condition. Thus, unless arc detection is inhibited during inverter startup and lamp ignition, the inverter may be prevented from properly starting and/or the ballast may be prevented from properly igniting the lamp. Additionally, although most lamps are capable, under ideal conditions, of igniting and operating normally within a short period of time (e.g., 20 milliseconds), some lamps, due to age or low temperature, require a much longer time to ignite and stabilize. Thus, arc detection should be inhibited for a period that is long enough (e.g., at least 200 milliseconds or so) to accommodate lamp starting under conditions that are less than ideal.
It is further desirable that a ballast possess some type of automatic restart capability wherein, within a specified time following detection of an arc condition and shutdown of the inverter, periodic attempts are made to restart the ballast and ignite the lamp. This feature is desirable in order to prevent a “latched” shutdown of the ballast (which necessitates that power to the ballast be turned off and then on again in order to reset the ballast) in the event of false detection due to a momentary power line transient or any of a number of anomalous phenomena (e.g., electrical noise) that pose no real threat to ballast reliability or safety. Also, because lamps are somewhat unpredictable, it is possible that an otherwise “good” lamp may sometimes fail to properly start on the first attempt. In such a case, a ballast with automatic restart capability will periodically attempt to start the lamp, rather than simply latching the ballast or its inverter in a shutdown state until such time as the power to the ballast is cycled.
For a ballast that powers multiple lamps and that includes automatic restart capability, in the event of a recurrent arc condition (i.e., an arc condition that continues to reoccur over an extended period of time such as, e.g., hours, days, weeks, months, etc.), the periodic (but unsuccessful) attempts to restart the ballast and ignite the lamps results in a regular (e.g., once per second) brief flashing of any remaining operational lamp(s). This regular brief flashing, which occurs on a sustained basis until either the arc condition is corrected or power is removed from the ballast, is considered to be visually annoying to occupants who are in the vicinity of the affected lighting fixture. Additionally, the periodic restart attempts are stressful to the components within the ballast. Thus, a need exists for an arc protection approach that not only minimizes visual annoyance to occupants, but that also avoids placing unnecessary stress upon the ballast components.
Yet another shortcoming of many existing approaches to arc protection is that those circuits often require a considerable amount of operating power. Typically, the operating power requirements increase with circuit complexity, especially when analog circuitry is extensively employed. Consequently, those circuits significantly detract from the overall energy efficiency of the ballast. Thus, a further need exists for an arc protection circuit that, in comparison with existing approaches, has relatively modest operating power requirements.
Ballasts with a current-fed self-oscillating inverter and a parallel resonant output circuit are currently the prevailing “instant start” design topology in North America. However, providing reliable arc protection within these types of ballasts presents a significant engineering challenge. In particular, many prior art approaches are susceptible to problems relating to arc detection resolution, and are therefore ill-suited for ballasts that power multiple (e.g., three or four) lamps. For example, in a ballast for powering three or four lamps and in the case of an arc condition that involves only one lamp or one lamp socket, any signal that is intended to be indicative of an arc condition may be “swamped out” by the fact that the remaining lamps and sockets are operating in a substantially normal manner. Because of this problem, one existing approach has been to provide a separate inverter and output circuit for each of the lamps powered by the ballast; such an approach has the obvious disadvantage of being quite expensive, especially for ballasts that power three or four lamps (in which case three or four separate inverters and output circuits are required).
The output wiring that is present between the output of the ballast and the lamp fixture introduces a certain amount of stray capacitance. That stray capacitance may impact the ability of an arc protection circuit to reliably detect the presence of an arc condition. Accordingly, a need exists for an arc detection circuit that is capable of accounting for any effects due to stray capacitances.
Thus, a need exists for a ballast having an arc protection circuit that is capable of reliably detecting an output arc condition (especially in the context of a multi-lamp ballast and in the face of stray capacitances due to wiring between the ballast and fixture(s), etc.). A need also exists for a ballast and arc protection circuit that provides a starting (i.e., inhibit) period in order to allow for proper lamp starting, as well as automatic restart capability in order to accommodate false detection and anomalous starting failure of a “good” lamp, but in a manner that minimizes visually annoying flashing and unnecessary stress to ballast components when an arc condition continues to reoccur over an extended period of time. A further need exists for an arc protection circuit with modest operating power requirements. A further need exists for an arc protection circuit that provides all of the aforementioned functional benefits, and that is readily and economically implemented within existing ballasts. Such a ballast and arc protection circuit would represent a considerable advance over the prior art.